1. Field of the Invention
The present invention relates to a head up-down mechanism in a printer for moving a thermal head into and out of contact with a platen of the printer for printing on a printing paper such as a thermal paper. Particularly, the invention is concerned with a head up-down mechanism in a printer suitable as an output device for a portable personal computer or the like.
2. Description of the Prior Art
Recently, as an output device for a portable personal computer or the like there has been used a small-sized thermal printer which makes printing directly on a printing medium with use of a thermal head. Such a thermal printer is required to be small-sized and is improved in performance while retaining a low cost. Particularly, a thermal head up-down mechanism is required to be an efficient mechanism because it exerts an influence on the printing speed in relation to the feed of printing paper.
Reference will be made below to a conventional thermal printer 24 having a head up-down mechanism.
In the conventional thermal printer 24, as shown in FIGS. 7 and 8, a back frame 26 located on a back side is formed with a platen mounting portion 28 for mounting a platen 27. At a lower position on the front side of the platen mounting portion 28 is disposed a support shaft 30 for supporting a carriage 29, the support shaft 30 being supported by side frames 31L and 31R in parallel with the platen 27. The carriage 29 is fitted on the support shaft 30 so that it can reciprocate along the platen 27.
On the front side of the support shaft 30 and in proximity thereto is disposed a carriage driving shaft 32 for driving the carriage 29, the carriage driving shaft 32 being supported rotatably by the side frames 31L and 31R and in parallel with the platen 27. A worm 33 for engagement with the carriage 29 is slidably splined to the carriage driving shaft 32. On a base frame 34 of the thermal printer 24 is disposed a rack plate 35, in parallel with the carriage driving shaft 32, the rack plate 35 having rack teeth 35a for mesh with the worm 33. To both ends of the carriage driving shaft 32 are connected power transfer gears 36, and driving gears 37a of a drive motor 37 are in mesh with the power transfer gears 36, so that a rotative driving force of the drive motor 37 is transmitted as a rotating force to the carriage driving shaft 32.
A worm fitting portion 39 for fitting therein of the worm 33 is formed in a concave shape in the lower edge portion of the carriage 29 and at a position close to the center thereof. Further, on the platen 27 side, two support portions 41 are projectingly provided on both lower corner portions of the carriage 29. The support portions 41 each have an insertion hole 40 for insertion therethrough of the support shaft 30.
According to this construction, with movement of the worm 33 on the carriage driving shaft 32, the carriage 29 moves while being supported by the support shaft 30.
On the platen 27 side, a fitting hole (not shown) is formed in an upper portion of the carriage 29, and one end portion of a coiled spring 42, which functions as an urging member for urging the thermal head 25 to the platen 27 side, is fitted in the fitting hole.
Between the carriage 29 and the platen 27, a flat plate-like thermal head mounting base 44 for mounting the thermal head 25 is fitted on the support shaft 30 pivotably between the support portions 41 of the carriage 29. The thermal head 25 is secured to the thermal head mounting base 44 at a position opposed to the platen 27, while in the side face on the carriage 29 side of the thermal head mounting base 44 is formed a fitting hole 45 for fitting therein of the opposite end portion of the coiled spring 42. With the urging force of the coiled spring 42, the thermal head mounting base 44 is turned about the support shaft 30, causing the thermal head 25 to come into pressure contact with the platen 27.
On the other hand, convex cam portions 48a and 48b are formed at both ends of the platen mounting portion 28 and at positions outside a printable area of the thermal head 25. The cam portions 48a and 48b project from the platen 27 side toward the carriage 29 side. The cam portions 48a and 48b respectively have front edges 46a and 46b in parallel with the moving direction of the carriage 29 and also have slant faces 47a and 47b which are inclined in the moving direction of the carriage 29 from the front edges 46a and 46b toward both end portions of the platen 27.
Consequently, when the thermal head 25 has moved beyond its printable area, the thermal head mounting portion 44, with movement of the carriage 29, moves along the slant faces 12a and 12b of the cam portions 13a and 13b against the urging force of the coiled spring 42 and causes the thermal head 25 to move away from the platen 27.
Now, a description will be given below of the operation of the conventional thermal printer 24 having the head up-down mechanism of the above construction.
When the driving force of the drive motor 37 is transmitted to the carriage driving shaft 32 through the power transfer gears 36. and causes the shaft 32 to rotate, the worm 33 starts rotating. Since the worm 33 is in mesh with the rack teeth 35a formed on the rack plate 35, it moves along the rack plate 35, and with this movement of the worm 33, the carriage 29 moves along the platen 27 while being supported by the support shaft 30.
Further, since the thermal head mounting base 44 is urged at all times toward the platen 27 by virtue of the coiled spring 42, the thermal head 25 secured to the thermal head mounting base 44 is brought into pressure contact with the platen 27 through a printing paper such as a thermal paper.
The thermal head 25, in its printable area, causes heat generating elements (not shown) to generate heat in accordance with a predetermined printing command issued from a control means (not shown), allowing printing to be performed for one line on the printing paper.
After this printing, when the carriage 29 moves as it is and goes beyond the printable area of the thermal head 25, the thermal head mounting base 44, with this movement of the carriage 29, moves up along the slant faces 47a and 47b of the cam portions 48a and 48b while turning toward the carriage 29 against the coiled spring 42, causing the thermal head 25 to be spaced away from the platen 27.
When the thermal head mounting base 44 goes up to the front edges 46a and 46b of the cam portions 13a and 13b, the carriage 29 stops and the printing paper is fed one line by means of a conveyance roller (not shown).
Once this paper feed is done, the drive motor 37 rotates in the opposite direction and this rotative driving force is transmitted to the carriage driving shaft 32 through the power transfer gears 36, whereupon the shaft 32 also starts rotating in the opposite direction and moves the carriage 29 in the opposite direction. In this way the same printing operation as above is repeated until a desired printing is completed.
Thus, in the conventional thermal printer 24 having the head up-down mechanism of the above construction, printing can be done by movements in two directions of the carriage 29, so that the effective printing speed can be improved.
In the thermal printer 24, however, there sometimes occurs a case where printing dot-formed positions become out of register due to the different moving directions of the carriage 29 and hence printing is not effected in a satisfactory manner.
According to the method adopted therein for solving such a problem, an energized position of the thermal head 25 is modified in accordance with a moving direction of the carriage 29 to thereby prevent the positional deviation of printing dots.
However, even with such correction of the energized position of the thermal head, it is difficult to make dot positions coincident completely in the bi-directional printing In a printer for a personal computer or the like requiring description of graphics, it is the usual way of doing that printing is performed in only one direction.
In this connection, when printing is performed in one direction by means of the conventional thermal printer 24 having the head up-down mechanism of the above construction, and at the time of insertion of printing paper or for paper feed after one-line printing, it is necessary that the thermal head 25 be allowed to strike on the cam portions formed at both ends of the platen 27 and be allowed to stand by in a head-up state.
More particularly, if one-way printing is conducted by means of the head up-down mechanism of the conventional thermal head 24 and where paper feed is done after printing, the paper feed is sure to be done after return to the stand-by position, so that the procedure of printing, carriage return, paper feed, and printing is repeated and it is impossible to conduct both paper feed and carriage return at a time, thus giving rise to the inconvenience that the effective printing speed decreases.